1. Field of the invention
The present invention relates to a method of micro-mechanically producing ultrafine silicon tips on a silicon base for the AFM/STM profilometry, e.g. of deep trenches in future semiconductor technology.
2. Description of the Prior Art
The scanning tunneling microscope (hereafter abbreviated STM) has stimulated the development of new techniques for microcharacterization of materials which are based on the use of a very fine tip. One of these techniques involves the atomic force microscope (hereafter abbreviated AFM) which has recently demonstrated the capability to profile and image conductors and insulators.
In the initial design of the AFM (Binnig G, Quate CF, Gerber Ch, (1986) Atomic Force Microscope, Phys. Rev. Lett. 56, 930-933 and EP-A-0 223 918) a sensor consisting of a spring-like cantilever which is rigidly mounted at one end and carries at its free end a dielectric tip profiles the surface of an object. The force between the object's surface and the tip deflects the cantilever, and this deflection can be accurately measured, for example by a second tip which is part of an STM. A lateral spatial resolution of 3 nm has initially been achieved.
Another version of the AFM includes optical detection instead of an STM detection. In this version a tungsten tip at the end of a wire is mounted on a piezoelectric transducer. The transducer vibrates the tip at the resonance frequency of the wire which acts as a cantilever, and a laser heterodyne interferometer accurately measures the amplitude of the a.c. vibration. The gradient of the force between the tip and sample modifies the compliance of the lever, hence inducing a change in vibration amplitude due to the shift of the lever resonance. Knowing the lever characteristics, one can measure the vibration amplitude as a function of the tip-sample spacing in order to deduce the gradient of the force, and thus, the force itself (Duerig U. T., Gimzewski J. K., Pohl D. W. (1986) Experimental Observation of Forces Acting During Scanning Tunneling Microscopy, Phys. Rev. Lett. 57, 2403-2406; and Martin Y., Williams C. C., Wickramasinghe H. K. (1987) Atomic Force Microscope-Force Mapping and Profiling on a sub 100-A Scale, J. Appl. Phys. 61(10), 4723-4729).
In the normal deflection mode of the cantilever beam, forces on the order of 10.sup.-12 N can be detected. The sensitivity of the sensor head can be further enhanced by vibrating the object to be investigated at the resonance frequency f.sub.o of the cantilever beam, as described by G. Binnig et al in Phys. Rev. Lett. 56 (1986), pp. 930-933.
A most critical component in the AFM is the spring-like cantilever. As maximum deflection for a given force is needed, the cantilever should be of optimum softness. Therefore, a cantilever beam with the inherent high sensitivity should have a minimum spring constant. In practice, for detecting positional changes on the order of &lt;0.1 nm, the spring constant C of the cantilever should be in the range from about 0.001 to 1 N/m. At the same time a stiff cantilever with a high eigenfrequency is necessary in order to minimize the sensitivity to vibrational noise from the building. Usually, ambient vibrations, mainly building vibrations, are of the order of &lt;100 Hertz. If the cantilever is chosen such that it has an eigenfrequency f.sub.o .gtoreq.10 kHz, the ambient vibrations will be attenuated to a negligible value.
For meeting both requirements, dimensions of the cantilever beam are necessary that one can only be obtained by microfabrication techniques.
Dimensions of an SiO.sub.2 cantilever beam compatible with C=0.034 N/m, and f.sub.0.sup.2 =21.1 kHz are for example: l1=250 .mu.m, w=9 .mu.m.degree., and t=1.6 .mu.m. A process for making cantilevers with these dimensions is described inter alia in J. Vac. Sci. Technol. A 6(2), Mar/Apr 1988, pp. 271-274, by Albrecht, T. R. and Quate, C. F., Atomic Resolution with the Atomic Force Microscope on Conductors and Nonconductors.
On the ends of such microcantilevers sharp tips have to be formed for profiling of the surface of an object. Should the structures to be profiled have a macroscopic depth exceeding for example 50 nm, the shape and the cross-section of the tip is decisive for the reproduction fidelity of the structure.